1. Technical Field
This invention relates generally to variable focus lenses, and particularly to a variable focus lens device having a focal point adapted to cyclical high-speed fluctuation to deepen the device's depth of focus.
2. Discussion
A variable focus lens device having a lens member and a separate a pressuring part adjacent to the lens member is disclosed, for example, in Japanese Patent Application Laid-Open No. Hei. 8-114703.
In this variable focus lens device, a pair of pressure-measuring strain gauges and a pair of temperature-compensating strain gauges are joined to the transparent elastic film for measuring the pressure of a transparent liquid acting on a transparent elastic film and controlling the pressuring part so that this pressure approaches a target value. These pressure-measuring strain gauges and temperature-compensating strain gauges form a Wheatstone bridge circuit, and distortion of the transparent elastic film is measured on the basis of an output from this circuit. Because the focal length of the lens member lens is temperature-compensated during this measurement, if the same measured values are used to control a piezoelectric unimorph driving the pressuring part, volume fluctuations of the transparent liquid resulting from thermal expansion and contraction are also compensated.
However, this related art variable focus lens device has a complex structure because the Wheatstone bridge circuit must be formed by a pair of pressure-measuring strain gauges and a pair of temperature-compensating strain gauges. Also, because the strain gauges must be mounted at correct angles in correct positions to form the Wheatstone bridge circuit, manufacturing is difficult to automate and is therefore labor-intensive.
Nevertheless, with a variable focus lens device having a transparent liquid sealed in a closed container and not having a temperature compensation function, the focal length of the lens changes and the focus position moves when the transparent liquid thermally expands or thermally contracts due to an increase in temperature resulting from a change in the surrounding temperature or extended operation. That is, it is sometimes unavoidably necessary for the device to be provided with a temperature compensation function, because without temperature compensation precise focus positioning sometimes is difficult to obtain, depending on the environment in which a variable focus lens device is to be used and the purpose of that use.